Question: The scale on a map is 10cm : 3km. If the distance between two cities is 6km, how far apart in cm are the two cities on the map?
The scale means that every 10cm on the map corresponds to an actual distance of 3km. An actual distance of 6km is the same as 2 $\cdot$ 3km. The distance between the two cities on the map is 2 $\cdot$ 10cm, or 20cm.